Lecture 23 Transport 2

Early discoveries about plant sugars came from studying aphids.
- Aphids pierce phloem, over-ingest sap and excrete sugary "honeydew" (also called "sundew").
- Researchers vaporised aphids and analysed this excretion to identify phloem composition.
- Illustrates: plants vigorously protect sugars; specialised herbivores evolve work-arounds.

Overall reaction (highly simplified):
6\,CO2 + 6\,H2O + \text{light} \;\rightarrow\; C6H{12}O6 + 6\,O2
Converts low-energy, oxidised carbon (CO₂) into high-energy, reduced carbon (carbohydrates).
Drives global carbon cycle, agriculture, biofuels, and ultimately most heterotrophic life.

Light Reactions
- Occur in chloroplast thylakoid membranes.
- Use photons + water to produce high-energy compounds:
  \text{ADP} \;\rightarrow\; \text{ATP}
  \text{NADP}^+ \;\rightarrow\; \text{NADPH}
- Release O_2 as a by-product.
Calvin Cycle (Carbon-Fixation Reactions)
- Stroma of chloroplasts; cyclic enzymatic pathway.
- Uses ATP and NADPH to reduce and assemble carbon.
- Net output (per two turns) = one C_6 sugar (e.g.
  glucose, fructose).

Standard "C₃ leaf" (majority of temperate species):
- Upper epidermis → palisade mesophyll (dense chloroplasts → high light capture).
- Spongy mesophyll (air spaces, gas exchange, water interface).
- Stomata concentrated on lower epidermis → minimise transpiration under direct sun.
First stable Calvin-cycle product = 3-carbon molecule → name "C₃".

Most abundant protein on Earth.
Catalyses addition of CO_2 to a 5-C acceptor (RuBP).
Fault: similar affinity for O_2 → photorespiration.
- When stomata close (hot/dry), internal CO2 ↓, O2 ↑.
- RuBisCO fixes O_2, generating toxic/energy-wasting products.

Kranz anatomy: concentric rings around vascular bundle.
- Outer ring = mesophyll (light reactions, initial CO_2 capture).
- Inner ring = bundle sheath (Calvin cycle).
First fixation by PEP carboxylase (insensitive to O_2).
- Forms 4-C acid (oxaloacetate → malate).
- 4-C acid shuttled to bundle sheath; decarboxylated → local CO_2 ‘pump’.
Benefits:
- High photosynthetic efficiency when stomata are partially/fully closed.
- Dominant in tropical/savanna grasses (maize, sugarcane).
Costs:
- Extra ATP needed to regenerate PEP and shuttle acids.
- Less competitive in cool, moist, high-CO_2 environments.
Has evolved independently ≈ \sim30 times across plant lineages.

Temporal separation rather than spatial.
- Night: stomata open; CO_2 fixed into 4-C malic acid and stored in vacuoles.
- Day: stomata closed; light reactions supply ATP/NADPH; malate decarboxylated → Calvin cycle runs internally.
Strong water saving; common in succulents, cacti, pineapple.
Trade-off: limited malate storage → lower maximum photosynthetic rate.

Rolled/needle-like leaves (Triodia, Spinifex, conifers):
- Inner "crypt" concentrates CO_2 and reduces vapor loss.
- High volume : surface ratio.
Thick cuticle, sunken stomata, reflective hairs and waxes.

Soil water potential ↑ → stomata open (no drought stress).
Leaf internal CO_2 ↓ (actively photosynthesising) → stomata open.
High light intensity (if water ample) → open.
High temperature (boosts respiration \Rightarrow CO_2 demand) → open until water stress overrides.
Circadian rhythms:
- Typical C₃/C₄: open day, close night.
- CAM: reverse.

Continuous living pipeline parallel to xylem.
Sieve-tube elements: elongated, no nucleus, sieve plates at ends.
Companion cells: sister cell (from same mother cell) with nucleus + abundant mitochondria; handles loading/unloading & metabolic control.
Associated parenchyma for storage/support.

Vascular cambium produces xylem inward, phloem outward.
In stems: phloem outer side; xylem inner.
In leaves: vein flips when entering blade → phloem faces lower epidermis (consistent with outside-of-stem position).

Source = tissue where \text{sugar production} - \text{utilisation} > 0.
Sink = tissue with net sugar demand.
Example summer tree:
- Leaves (source) → roots, fruits, growing shoots (sinks).
Example early spring deciduous tree:
- Roots/stem starch → hydrolysed to sucrose (source).
- Buds & developing leaves (sink).
- Demonstrates bi-directional sap flow potential.

Sugar maple aggressively mobilises root reserves at thaw.
Sap exudes rapidly when trunk is tapped → maple syrup industry.
Ecological angle: rapid leaf flush out-competes slower neighbours but increases vulnerability to sap feeders.

Active loading of sucrose into source sieve tubes (uses ATP in companion cells).
Raises osmotic concentration → lowers water potential.
Water moves in from adjacent xylem by osmosis → builds positive pressure (turgor).
Pressure gradient pushes sap through sieve plates toward sinks.
Active unloading at sink removes sucrose; local water potential rises.
Water exits back to xylem → recycled up to leaves.

Phloem chooses pathway with greatest pressure differential (fastest unloading sink).
- Rapidly growing fruit may out-compete a slower-growing branch tip.
No conscious choice—purely emergent from physical laws & metabolic rates.

Understanding photosynthetic pathways guides crop breeding (e.g., engineering C₄ traits into rice for drought resilience).
Improved phloem knowledge aids pest management (aphid control) and nutrient optimisation.
Maple syrup industry = direct exploitation of phloem dynamics.
Global climate change (lower atmospheric CO_2 historically vs present) highlights evolutionary constraints on RuBisCO efficiency.

Photosynthesis captures solar energy; phloem distributes it as chemical energy.
Evolution produced anatomical (C₄), temporal (CAM), and structural (rolled leaves, needles) solutions to RuBisCO’s oxygen dilemma and water scarcity.
Phloem transport is an energy-assisted, pressure-driven conveyor from sources to dynamic sinks, reversing direction seasonally.
Coupling of xylem & phloem allows plants to recycle water while moving sugars, elegantly meshing physical physics with biological control.